I am finding this a big issue also. Currently running a DIA campaign and using the online player.
I find that the volume between moods can be quite off. For example, I have to turn up the master volume for taverns, and down for battle music. (I know there should be a difference, but the difference can be quite big!).
I also find that the oneshot elements really get drowned out, especially in battle.
I think one way to solve this would be to have an overall volume for each mood (that isnt the overall top volume). That way we could dampen down one mood, whilst keeping another higher, and could keep the one shots the highest.
Also, it would be great if this could be done through API for integration into other stuff also.